The present invention relates to fluoropolymer coated fasteners, and, more particularly, to a new process for effectively and efficiently coating preselected portions of threaded fasteners with a fluoropolymer.
It has been recognized for some time that threaded fasteners may be protected from thread contaminants by coating the threads with fluoropolymer resin. Typical contaminants that may interfere with proper threaded coupling of the fasteners include paint, anti-corrosion primers, weld spatter and solder. Coating the fastener threads with a fluoropolymer before exposure to these contaminants, reduces or prevents the contaminants from adhering to the fastener. In the use of such fluoropolymer coatings, however, it is important, and often critical, that the fluoropolymer coating be applied only to selected portions of the fastener. Indiscriminate application of the coating over all areas of the fastener is to be avoided. Examples of prior art teachings in this field are found in U.S. Pat. Nos. RE33,766 and 5,221,170. The disclosures of these patents are incorporated herein by reference.
Although the processes and coated fasteners as disclosed in the above identified patents have achieved substantial commercial success, they nonetheless suffer from certain disadvantages. For example, in the practice of this prior art the fasteners are heated prior to application of the fluoropolymer powder. As a result, the fasteners are necessarily heated to a temperature substantially above the fluoropolymer melting point to accommodate some cooling of the fastener during transit from the heating station to the powder spray station. This elevated temperature, in the range of about 750xc2x0 to 900xc2x0 F., can damage certain fastener materials or platings, thus, limiting the applicability of the prior art technology.
Another disadvantage associated with the prior art is that relatively large amounts of fluoropolymer powder are required to achieve a generally uniform and continuous coating, thereby raising the cost of the process.
Another disadvantage with the prior art is that, traditionally, fluoropolymer coatings are baked and sintered for extended periods of time, increasing processing time.
Initial experiments were conducted some time ago in an attempt to electrostatically deposit fluoropolymer powders using conventional corona charging techniques. However, the resulting fluoropolymer powder coating was indiscriminately applied onto a wide area of the fastener, requiring some form of masking to limit the coating to only the preselected areas where the coating was desired. Additionally, when attempting to coat internally threaded fasteners, Faraday cage effects come into play, which further limits the integrity of the resulting coating. The possibility of electrostatically depositing the powder by corona charging techniques was therefore rejected since masking would prove too difficult and costly in high volume production.
There is, accordingly, a need for a new fluoropolymer coating process that employs lower temperatures, less fluoropolymer resin and is less costly; while maintaining the benefits and advantages of the known powdered fluoropolymer application technology.
The present invention is directed to a process for the application of fluoropolymer to a preselected area of a threaded fastener, and particularly to substantially all of the threads of the fastener.
The fluoropolymer is supplied to a spray nozzle in powder form and is subjected to a triboelectrostatic charging process so that individual particles discharged from the spray nozzle are electrically charged. In the preferred form of the invention, the fluoropolymer powder is triboelectrically charged, entrained in an air stream discharged from the nozzle and directed onto the preselected area of the fastener. In this manner a generally uniform powder coating is deposited onto the preselected area of the fastener while the fastener is maintained at room temperature. Thereafter, the fastener is heated to a temperature above the melting point of the fluoropolymer to thereby coalesce the deposited powder into a continuous film coating which adheres, upon cooling, to the pre-selected area of the fastener.
The process of the present invention may be used with either internally or externally threaded articles, such as internally or externally threaded fasteners. In accordance with one preferred embodiment, an externally threaded fastener is heated in a manner which raises the temperature of only a preselected area of the fastener to the fluoropolymer melting point. This preferred heating technique minimizes the retention of fluoropolymer inadvertently deposited on areas of the fastener other than the preselected area, and allows this undesired fluoropolymer to be easily removed, even after heating.
Using the present invention, the coating of internally threaded fasteners may be confined to the threaded area only and, therefore, the entire fastener may be heated to coalesce the deposited powder.
With the present invention, heating times required for fluoropolymer adherence may be substantially decreased.